This application pertains to the art of electroplating and more particularly to high current density deposition of electroplate. The invention is particularly applicable to the electrodeposition of lead-tin alloys on sleeve bearings and will be described with particular reference thereto. It will be appreciated, however, that the invention has broader applications including the electrodeposition of other metals and alloys onto other workpieces.
In high current density depositions of electroplate, the current density is proportional to the square root of the relative movement between the electroplating solution and the workpiece. Heretofore, high current density depositions of electroplate have been achieved by moving the workpiece relative to the plating solution or by moving the plating solution relative to the workpiece. To plate sleeve bearings by moving them relative to the solution, gives rise to many problems. To withstand the rotational forces encountered when spinning a column of bearings about an anode, secure holding devices were necessary. Such holding devices tended to make loading and unloading of workpieces difficult and time-consuming. Further, these holding devices needed to be dynamically balanced to spin smoothly. In addition to the mechanical problems encountered in the rotating holding devices, the rotation caused churning of the plating solution. This churning required that the plating cell be totally enclosed to prevent the solution from splashing out of the cell and to prevent air from being entrained in the plating solution and oxidizing the plating chemicals. Such total enclosure of the plating cell further hindered loading and unloading operations.
Moving the plating solution relative to the workpiece required moving a large volume of solution through the plating fixture. Typically, electroplating a ten-inch inside diameter bearing surface 26 inches long with a current density of 800 amperes per square foot required 1750 gallons per minute of solution to be pumped between the anode and the workpiece. Problems arose in pumping this large quantity of highly corrosive plating solution through this small volume. The high pressures necessary to move the plating solution required elaborate holding devices to hold the bearings securely in place. These holding devices again tended to be difficult to load and unload. Further, these high pressures tended to compound the difficulties in loading and unloading the workpieces and to entrain air in the plating solution.
The prior art high current density electroplating cells commonly used either a solid, soluble anode or an insoluble anode. A primary problem with soluble anodes in high current density systems is that they are dissolved quickly. For example, electroplating a ten-inch inside diameter bearing surface 26 inches long with a current density of 800 amperes per square foot, dissolves 371/2 pounds of lead-tin per hour from the anode. This is the equivalent to a standard two-inch diameter anode.
A principal problem with insoluble anodes is that they degrade the electroplating solution. The insoluble anodes liberate oxygen which destroys some of the constituents of the plating solution. Further insoluble anodes are not truly insoluble but rather small amounts of contaminant metals are dissolved and suspended in the plating solution.
The present invention overcomes these problems and others while it also provides a high current density electrolytic deposition system which is practical for production use.